42. Operator Overloading

Functions marked operator with conventional names let your types work with built-in operators. plus backs +, times backs *, and get backs the [] indexing syntax.

data class Vec(val x: Int, val y: Int) {
    operator fun plus(other: Vec) = Vec(x + other.x, y + other.y)
    operator fun times(scale: Int) = Vec(x * scale, y * scale)
    operator fun get(index: Int) = if (index == 0) x else y
}

fun main() {
    val a = Vec(1, 2)
    val b = Vec(3, 4)

    println(a + b)        // calls plus
    println(a * 3)        // calls times
    println(a[0])         // calls get
}

a + b compiles to a.plus(b), a * 3 to a.times(3), and a[0] to a.get(0). The operator names are fixed, so the compiler knows which symbol each one maps to.

Running it:

$ kotlin run
Vec(x=4, y=6)
Vec(x=3, y=6)
1

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